Metal-Assisted Chemistry for Anti-Reflection Etching of Silicon

The global market for solar technology is expected to grow by 21% in 2016 to reach a total of 66 GW. With demand for solar cells increasing, there is a renewed focus on improving energy conversion efficiency. One major problem for energy conversion in solar cells is the loss of solar energy due to the high refractive index of silicon. An effective way to reduce the reflective property of silicon is through surface texturing, or etching, which may improve the amount of photon absorption in the cell and therefore either increase its power output, or enable the use of thinner (and therefore less-expensive) devices.

NREL scientists have patented methods to etch the Si wafer to have nanoscale pores at room temperature. These pores help angle light reflected from the Si surface into the cell, increasing the rate of exciton generation. This decrease in overall surface reflectivity makes the silicon wafer appear to be black. This Black Silicon has been proven to reduce the reflectivity level of silicon from the standard 11% (with ARC) all the way down to 2%.

Description

While previous attempts at etching Black Silicon surfaces have used Au and Ag nanoparticles, researchers at the National Renewable Energy Lab, NREL, have developed a way to utilize copper when etching a nanoporous black silicon surface. Using Cu to etch silicon is advantageous because Cu is orders of magnitude less expensive than Au or Ag, is more soluble when mixed with silicon, and is already compatible with standard industrial processes used in Si microfabrication facilities. Moreover, while Au and Ag nanoparticles can also be detrimental to the cell due to their diffusion potential, this problem is eliminated by use of Cu nanoparticles.

Benefits

Using copper to etch nanoporous silicon reduces costs

Compatible with existing industrial technologies

Using copper is less detrimental to the silicon solar cell than using gold or silver

Copper-assisted, anti-reflection etching of silicon surfacesA method (300) for etching a silicon surface (116) to reduce reflectivity. The method (300) includes electroless deposition of copper nanoparticles about 20 nanometers in size on the silicon surface (116), with a particle-to-particle spacing of 3 to 8 nanometers. The method (300) includes positioning (310) the substrate (112) with a silicon surface (116) into a vessel (122). The vessel (122) is filled (340) with a volume of an etching solution (124) so as to cover the silicon surface (116). The etching solution (124) includes an oxidant-etchant solution (146), e.g., an aqueous solution of hydrofluoric acid and hydrogen peroxide. The silicon surface (116) is etched (350) by agitating the etching solution (124) with, for example, ultrasonic agitation, and the etching may include heating (360) the etching solution (124) and directing light (365) onto the silicon surface (116). During the etching, copper nanoparticles enhance or drive the etching process.